A dye-sensitized solar cell was first developed by M. Gratzel, et al. [B. O'Regan, M. Gratzel, Nature, 1991, 737, 353] and its metal oxide semiconductor electrode comprises a photosensitive dye capable of absorbing visible light to generate electron-hole pairs and nanocrystalline titanium oxide particles, an n-type semiconductor, that can transfer the generated electrons, the dye being regenerated by an electrochemical oxidation-reduction reaction of “I−/I3−” contained in a liquid electrolyte, thereby generating an electric current.
Such a dye-sensitized solar cell may possibly be manufactured in a more cost-effective manner than other solar cells such as a monocrystalline silicon solar cell, an amorphous silicon solar cell, and a chemical compound semiconductor solar cell, and, for that reason, it has been regarded as a next-generation solar cell. However, the photo-energy conversion efficiency of the conventional dye-sensitized solar cell is lower than those of the silicon solar cell and the chemical compound semiconductor solar cell.
One of the methods to solve the above-mentioned problem is to enhance the light absorption efficiency of a metal oxide layer to obtain a high short-circuit current density (Jsc), e.g., by increasing the amount of a dye adsorbed on the metal oxide layer so that the dye can efficiently absorb incident light. In a typical dye-sensitized solar cell, the metal oxide layer is made porous to have an increased surface area. Such a porous titanium oxide layer is prepared by dispersing in ethanol 10 to 50 nm-sized anatase crystalline titanium oxide particles produced by hydrothermal synthesis of titanium alkoxide, adding an organic polymer or oligomer binder to produce a paste, and coating the paste on a transparent conductive substrate, followed by sintering. This process can be performed by various thin film formation methods such as roller coating, air knife coating, blade coating, wire bar coating, slide hopper coating, extrusion, curtain coating, spin coating, spray coating, offset coating, gravure printing, screen printing, and wet printing [Japanese Patent Publication No. 2006-286528A.]
Further, in order to maximize the light absorption by increasing the number of light absorbing channels, a titanium oxide precursor is treated with a surfactant to form a mixture using a sol-gel process, and then, the mixture is subjected to a hydrothermal reaction to form a scattering layer composed of porous titanium oxide particles having a particle size of several tens to several hundreds of nanometers [Journal of Colloid and Interface Science, 2007, 316, 85-91.]
However, the conventional methods using an organic binder have a problem in that the short-circuit current density deteriorates due to the increases in the interfacial inter-particle resistance and the particle-substrate interface resistance. Further, after the organic binder is removed by drying and sintering procedures, the resulting titanium oxide layer often undergoes cracking, and the inter-particle resistance increases. Moreover, the titanium oxide layer formed by the conventional method has a very dense structure and a low porosity so that a high-viscosity electrolyte such as a solid or quasi-solid electrolyte cannot easily penetrate the titanium oxide layer, resulting in a drastic decrease in the photoelectric conversion efficiency.
There has recently been developed an electrospray method for forming a porous titanium oxide layer composed of nanocrystalline titanium oxide particles. Such an electrospray method has been used in the field of electrostatic coating, to form a compact and uniform film on a substrate by the electrostatic force in a high-voltage electric field. For example, a uniform nanoparticle layer may be formed on a substrate by electrospraying a metal oxide nanoparticle dispersion on a substrate in a high-voltage electric field [see J. Electrochemical Society, 2006, 153(5), A826-A829.] However, the nanoparticle layer prepared by this method has a high density, a low porosity and a small pore size (about 10 nm or less), so that a high-viscosity electrolyte cannot easily penetrate the resulting nanoparticle layer, leading to an extremely low photoelectric conversion efficiency when used in a gel-type electrolyte or a solid state electrolyte.
A plastic substrate is preferred as a substrate for a semiconductor electrode in the preparation of a dye-sensitized solar cell due to its flexibility and light weight. However, when a plastic substrate is used, high-temperature sintering cannot be performed so that a metal oxide layer coated on a substrate becomes easily detached from the substrate and its electron transport efficiency becomes low. Further, the metal oxide particles incompletely bonds together, thus decreasing the photoelectric conversion efficiency.
Therefore, it is required to develop a metal oxide layer having a high specific surface area, a high porosity, and a large pore size, which has excellent contact properties even after a low-temperature sintering process and has a low interfacial resistance, a high electron transport efficiency, and a high photoelectric conversion efficiency, even when a gel or solid electrolyte is used.